RE: [AVT] -Bursty packet loss for text. - Was: How is SDP a=fmtp used for RFC 2198?

Bursts of consecutive loss are, as you suggest, relatively infrequent and
are usually due to link failures (see paper from Boutremans).  Sparse bursts
are however very common - typically up to 1-2 seconds of 20-30% loss density
(see www . voiptroubleshooter.com/voiptr_burstloss.htm)

Alan Clark
Telchemy

-----Original Message-----
From: avt-admin@ietf.org [mailto:avt-admin@ietf.org]On Behalf Of Gunnar
Hellstrom
Sent: Friday, April 09, 2004 1:16 AM
To: Randell Jesup
Cc: avt@ietf.org; paulej@packetizer.com; magnus.westerlund@ericsson.com;
Colin Perkins
Subject: RE: [AVT] -Bursty packet loss for text. - Was: How is SDP
a=fmtp used for RFC 2198?

You said:
> This assumes totally independant loss.  The reality is
> that loss is
> pretty strongly _not_ independant; i.e. the conditional probability of
> losing packet N+1 after losing packet N is high, and typically way higher
> than the base loss probability p (probability of losing packet N
> if packet
> N-1 was received).

The text packetization avoids loading the network and avoids bursty packet
loss by recommending 300 ms between transmissions. So, you collect typed
characters during 300 ms and then form a packet and transmit. If nothing is
typed, nothing is transmitted.

The observations on bursty packet loss pattern you refer to are made on
audio transmission, where packets are sent with 20-30 ms intervals. That
gives a very different probability for influence by bursts of packet loss.
Figures for common lengths of bursts I have seen have indicated that bursts
over 50 ms are rare.

So, I feel quite safe with the calculation being reasonably accurate
without taking burstiness into account.

Regards
-------------------------------------------
Gunnar Hellstrom
Omnitor AB
Renathvagen 2
SE 121 37 Johanneshov
SWEDEN
+46 8 556 002 03
Mob: +46 708 204 288
www.omnitor.se
Gunnar.Hellstrom@Omnitor.se
--------------------------------------------

> -----Original Message-----
> From: avt-admin@ietf.org [mailto:avt-admin@ietf.org]On Behalf Of Randell
> Jesup
> Sent: Thursday, April 08, 2004 9:32 PM
> To: Gunnar Hellstrom
> Cc: Colin Perkins; magnus.westerlund@ericsson.com;
> paulej@packetizer.com; avt@ietf.org
> Subject: Re: [AVT] How is SDP a=fmtp used for RFC 2198?
>
>
> "Gunnar Hellstrom" <gunnar.hellstrom@omnitor.se> writes:
> >3. Each generation of redundant info gives quite good
> reliability increase.
> >
> >Look at this calculation:
> >
> >For RFC2793bis text transport in text form, this is the theory:
> >
> >Assume that the risk of loosing a packet is p
> >The risk of loosing also next text packet is also p
> >
> >The bad case is only when both are lost. The likelihood for
> that is p*p .
> >
> >You can check with 50% loss and sending two packets. There are
> totally four
> >outcomes.
> >
> >lost -not lost
> >lost - lost
> >not lost - lost
> >not lost - not lost
> >
> >Only one out of four is unsuccessful. that is 25%.
> >
> >0.5 * 0.5 = 0.25 the probability calculation is valid.
> >
> >If you add one more redundancy level, the likelihood of all
> three lost is
> >p*p*p
> >
> >With real figures, 20% packet loss and three transmissions give
> 0.2 * 0.2 *
> >0.2 = 0.008 =0.8%total loss.
>
>         This assumes totally independant loss.  The reality is
> that loss is
> pretty strongly _not_ independant; i.e. the conditional probability of
> losing packet N+1 after losing packet N is high, and typically way higher
> than the base loss probability p (probability of losing packet N
> if packet
> N-1 was received).  I.e. bursts are not an unusual type of loss.
>
>         From numbers I've seen, the conditional probability for a second
> packet to be lost (loss burst size >= 2) on the internet is around 15% or
> more (with a p~= 1%).
>
>         Of course these values are way lower than the 50% or 20%
> mentioned
> above, but they're also heavily dependant on the physical links
> traversed.
> (a 3GPP link may be much worse).
>
> --
> Randell Jesup, Worldgate Communications, ex-Scala, ex-Amiga OS team
> rjesup@wgate.com
> "The fetters imposed on liberty at home have ever been forged
> out of the weapons
> provided for defence against real, pretended, or imaginary
> dangers from abroad."
> 		- James Madison, 4th US president (1751-1836)
>
>
> _______________________________________________
> Audio/Video Transport Working Group
> avt@ietf.org
> https://www1.ietf.org/mailman/listinfo/avt
>
>

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Audio/Video Transport Working Group
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https://www1.ietf.org/mailman/listinfo/avt

_______________________________________________
Audio/Video Transport Working Group
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https://www1.ietf.org/mailman/listinfo/avt